This class exposes two functions. `evalNode` take an AST node and evaluates it down to simplest terms. `eval` takes an array of AST nodes and calls evalNode on each of them. I don’t think we’re quite ready to be producing HTML yet so let’s not get ahead of ourselves.

Currently I'm relying on JavaScript's semantics
around truthiness. I think I'm going to retain
that ease of use (i.e. `(if list #t #f)` should
return true if the list has more than 0 elements,
etc) but I want to specify that behavior myself.

It's a little hacky and not very clean but it's
working for now.

No variables or conditionals or anything yet, just
the basic ability to turn s-expressions into HTML.